Method of electrodepositing microcrack chromium coatings



Unite States Patent 3,388,il49 METHUD 0F ELECTRGDEPGSZTENG MTCRU- CRACKCHROMHJM COATINGS Gatan dc Coye de Castelet, Eiliancourt, France,assignor to Rcgie Nationale des Usines Renault, Eillanconrt, Seine,..rance No Drawing. l iied Mar. 15, 1965, Eier. No. 440,019 (Claimspriority, application France, Oct. 12, 1964,

991,133, Patent 1,447,976; Nov. 24, 196 5, 996,104;

Jan. 27, 18 65, 3,439

Mr Ciaims. (Cl. Edd -29) ABSTRACT 0 F THE DISCLGSURE Chromium platingwith increased resistance to corrosion and flaking is produced by firstelectrodepositing on the substrate a nickel layer with a high internalstress level, sufiicient to produce micro-cracks therein during theelectrodepositing of th chromium layer, and then electrodepositing thechromium layer. The electrolyte bath from which the cracking nickel iselectrodeposited preferably includes (a) nickel chloride, nickelsulphamate or nickel fiuoborate and (b) acetic acid ammonium acetate ornickel acetate, or mixtures of them.

So-called decorative chromium deposits usually comprise, by way ofterminal deposits, a deposit of nickel followed by a deposit ofelectrolytic chromium.

For many years it was thought that the presence of the chromium platingdid not affect the overall protection but served only to improve theeye-appeal of the parts treated in this way.

More recently, an examination of anomalies noted in the resistance tocorrosion of chromium-plated articles has led to the conclusion that thechromium plating can, on the contrary, greatly affect the protection,and that this role is not one of providing added protection but insteadof speeding up the corrosion to a greater or lesser extent, depending oncircumstances.

The physical condition of the chromium plating plays a predominant part.The metal may be deposited in the form of a tight coating, but becauseof the brittleness of electrolytic chroming, this tightness is destroyedfairly soon owing to the appearance of cracks or holes. Moreover,immediately after it is deposited, the chromium may be porous to someextent or embody minute cracks in a more or less large number per unitarea, resulting in the formation of. individual elements placed more orless closely together.

It has been found that the depositing of chromium speeds up thecorrosion of the nickel all the more as the density of the porosities orcracks is smaller; conversely, a large number of porosities or cracksresults in a very slow nickel corrosion rate.

This fact has been used as the basis for certain methods proposed forimproving the overall resistance of chromium-plated parts to corrosion.For instance, one way of achieving this is to obtain chromium with amicrocrack structure (cracks spaced from one another by a few tenmicrons). This can be obtained either by a double-coating chromingprocess, which requires chromium thicknesses in excess of 0.5 micron, orby a single-coating chroming process in chromium baths containing asmall quantity of selenium, in which the microcrack structure isproduced with a chromium thickness of approximately 0.3 micron.

Another method aiming at obtaining microporous chromium depositsconsists in applying, onto the terminal nickel coating (usually brightnickel), a thin nickel coat- 3,388,649 Patented June 11, 1968 ing aboutone micron thick containing non-conductive particle inclusions. Thechromium coating deposited on this nickel is itself deposited in .porousform due to the presence of these non-conductive particles.

The present invention has for its object to provide a method ofobtaining microcracked chromium under unusually simple, readilyapplicable conditions. This method consists in depositing on the metal,for instance on the nickel normally constituting the ultimate protectivecoating prior to the chromium coating, a thin metal coating underconditions such as to cause the metal to be highly stressed and barelyductile. During the subsequent chroming operation, this last coating ofmetal and the coating of chromium undergo simultaneous microcracking,thereby imparting excellent resistance to corrosion to the objecttreated in this way.

The thinner the coating of this intermediate metal deposited torsubsequent cracking, the more advantageous it will be for itselectrochemical potential to lie between that of the subjacent metal andthat of the terminal chromium plating (in the state of surface oxidationto which it is normally brought after a few hours or a few days haveelapsed). By thin coating is to be understood a coating whose thicknessis included between a fraction of a micron and approximately 3 or 4microns.

Whilst various metals, or alloys based on nickel or cobalt inparticular, can be employed, a coating of nickel is entirely suitablefor the deposit designed to become cracked, provided its potential isslightly higher than that of the previous metal coating. Furthermore,this intermediate coating must not detract from the final appearance ofthe chromium-plated part.

Two distinct processes for obtaining a chromium deposit of microcrackedstructure according to the principle of the present invention will nowbe more particularly described by way of non-limitative examples of thesubject method of this invention. It is, however, to be clearlyunderstood that the examples given hereinafter do not describe suchpreliminary operations involving degreasing and depositing metals suchas copper, brass, zinc, levelling nickel, and so fourth, as may beconsidered useful for reasons unconnected with the obtainment of themicrocracking.

Example 1 Principal protective nickel plating, for example bright nickelplating Rinsing operations Depositing of the cracking nickel under thefollowing conditions:

Optimum Anhydrous nickel acetate, 50 g./l. to

saturation 100 g./l. Hydrated nickel chloride, 100 g./l. to

saturation 375 g./l. pH, 2 to 6 5 to 5.5. Current density, 1 to 15a./dm. l0 a./dm. Duration, 15 sec. to 2 min. sec. Temperature (below C.)20 to 25 C.

Agitation optional (with compressed air, say) Rinsings Chromium platingRinsings Drying Example 2 for example) Rinsings 3 Depositing of crackingnickel, in the following solution:

Optimum Hydrated nickel chloride, 100 g./l. to

saturation 400 g./l. Anhydrous ammonium acetate, 50 g./l.

to saturation 100 g./l. pH, 2 to 6 to 5.5. Current density, 1 to 15a./dm. 10 a./dm. Duration, sec. to 2 min. 30 sec. Temperature (below 35C.) to C.

Agitation optional (using compressed air, say) Rinsings Chromium platingRinsings Drying The cracking-nickel-plating baths used in the conditionsspecified in the two foregoing examples give, for plating times of lessthan two minutes, a coating of brightness such as to ensure thesubsequent obtainment of very bright chromium plating.

In certain special cases, however, it may be necessary to extend theduration of the cracking-nickel-plating operation for a considerablygreater time. This may be the case, for instance, when the parts to beprotected have heavily recessed portions, since the thickness willincrease less rapidly in such places than on the raised portions. It mayalso happen that certain installations, by their very design, will notpermit of reducing the duration of the crackingnickel-plating to 2minutes or less.

It is then necessary to add to the bath one or more suitable brighteningsubstances. Such brightening substances must not appreciably aiiect thepotential of the cracking-nickel deposit, so that the electrochemicalprotective process may be maintained as described precedingly. Nor mustthey reduce the propensity for cracking of the ultimate coat of nickelwhich generates the microcracking in the subsequent chromium plating.

By way of brightening agents it is possible to utilize the sulfonates,the sulfonamides and the sulfonimides of aromatic compounds. Morespecifically, the sodium salt of sulfonimide ortho-benzoyl acid(saccharine) can be added in proportions ranging from 0.5 to 3 g./l.,the optimum being around 1 g./liter, through this is by no meanscritical.

If added to the cracking-nickel-plating baths, butyne 1-4 diol willenable very bright deposits of cracking nickel to be obtained,regardless of the thickness of the deposits. This compound is added tothe bath in the proportion of 0.1 to 3 g./liter.

Other brightening agents which will enable particularly brightcracking-nickel deposits to be obtained, regardlessof their thickness,are the amino derivatives of heterocyclic compounds such as2-aminothiazole.

Lastly, when cobalt is added to the cracking nickel baths, it alsoenables the brightness of the deposit to be enhanced for concentrationsranging from 1 to g./ liter.

The following compositions and procedures may be mentioned by way ofcomplementary examples of the depositing of cracking nickel according tothe invention:

Example 3 Bath with addition of butyne 1-4 diol: Optimum Anhydrousnickel acetate, g./l. to

saturation 100 g./l. Hydrated nickel chloride, 100 g./l. to

saturation 375 g./l. Butyne 1-4 diol, 0.1 g./l. to 3 g./l. 0.2 g./l. pH,2 to 6 5 to 5.5. Current density, 1 to 15 a./drn. 10 a./dm.

Duration, 15 sec. to 5 min. Temperature (below 35 C.) 20 to 25 C.

4 Optional agitation (with compressed air or by displacing thecathodes).

Example 4 Bath with addition of Z-amino-thiazole: Optimum Anhydrousnickel acetate, 50 g./l. to

saturation g./1. Hydrated nickel chloride, lOO g./l. to

saturation 375 g./l. pH, 2 to 6 4 Z-arnino-thiazole, 5 mg./l. to 100mg./l l0 mg./l. Current density, 1 to 15 a./dm. 10 a./drn. Temperature(below 35 C.) 20 to 25 C.

Optional agitation (with compressed air or by displacing the cathodes).

Example 5 Optional agitation (with compressed air or the cathodes).

by displacing The thickness of the cracking-nickel deposit can beconsiderably increased by the aid of the brightening agents utilized inaccordance with the invention. This enables the method to be used inexisting installations with minimum or no modification.

Indeed, this leads to the possibility of a simplified application: forwhen the electrolysis baths ermit of obtaining bright and crackingdeposits of a thickness appreciably greater than a few microns, forinstance 10 to 20 microns, then in accordance with the present inventionthese deposits can be substituted for the terminal protective nickelcoating instead of being superimposed thereon.

The following may be mentioned by way of non-limitative examples:

(a) The sequences customarily consisting of a superimposed copper (orbrass) deposit and a conventional nickel deposit, in which the latterdeposit is replaced by thick bright deposit of cracking nickel or nickelalloy.

(b) The sequences comprising a coating of dull or levelling nickelfollowed by a coating of bright nickel, this double nickel coating beingobtained by the so-called binickel" or duplex method, in which thecustomary coating of bright nickel is replaced by a thick bright coatingof cracking nickel.

It is to be noted that this operating sequence permits of combining theadvantages offered by duplex nickel plating and microcrackchromium-plating; moreover, this sequence can be executed without theneed to seek a more noble cracking nickel than the subjacent coating oflevelling nickel.

Lastly, in accordance with the present invention, in the case of certainbright or scmibright nickel-plating baths in which the additives mayresult in a degree of surface passiveness, perfect adhesion of thecracking-nickel deposit is ensure by initially depassivating the brightor semibright nickel deposit. This is achieved by means of the followingoperations:

Careful rinsing of the bright or semibright nickel deposit;

Treatment in an alkali cyanide-base depassivation bath, for which thefollowing formulation represents a nonlimitative example:

Caustic soda 45 g./l. Anhydrous sodium carbonate 45 g./l. Sodium cyanide20 g./l. Temperature: Ambient Current density 5 to a./dm. Duration ofcathode pass to 30 sec.

Careful rinsing before insertion into the bath, thereby ensuring adeposit of cracking nickel.

In cases where the initial nickel deposit is strongly passivated,provision may be made for an anode pass lasting approximately 5 secondsprior to the cathode pass.

While cracking-nickel-plating baths as a whole are no more sensitive toimpurities than nickel-plating baths in general, recourse may be had, inorder to maintain such baths in good condition, to intermittent orcontinuous filtering, or else to some method well known per se such aselectrolysis on low-current-density cathodes for eliminating foreignmetal, or neutralization and filtering on active charcoal in order toget rid of undesirable organic substances and the like.

It should be noted that the deposit of cracking nickel cracks slowlyeven in the absence of subsequent chromium plating. It is thereforenecessary to avoid unduly long waits between the deposit of crackingnickel and the chromium plating, for otherwise the final chromiumplating would mask the cracks already formed in the nickel, which wouldrun counter to the aim sought.

This delay will vary with operating conditions and with the structureand stress specific to the subjacent nickel deposit. By way ofindication, however, it may be stated that spontaneous cracking in theultimate nickel deposit egins after approximately a quarter of an hourhas elapsed.

By applying the method according to the invention, the microcrackedstructure can be developed over all the chromium-plated surfaces, usingconventional chromiumplaiing baths comprising, as catalysts, thesulphuric, hydrofluoric, fluosilicic, and like ions.

It must be added that the use of special chromiumplating bathscontaining, say, selenium or additive particles insoluble in theintermediate-deposit solution, while not bringing marked benefits to themethod, is not incompatible with the use of the cracking depositsspecified hcreinabove.

Vlhat is claimed is:

1. A method of depositing a corrosion resistant microcracked coating ona substrate which comprises electro depositing nickel on said substratefrom an aqueous acidic bath and thereafter electrodepositing chromium onsaid nickel, said nickel being deposited with a high internal stresslevel, said stress level being suit'icient to produce micro-crackstherein during the deposition of the chromium layer which in turnproduces a micro-crack pattern in the chrom um layer.

2. The method in accordance with claim 1, in which the said deposit ofnickel is applied to a previous metallic deposit.

3. The method in accordance with claim 2, in whizh the deposited nickelhas an electrochemical potential between that of the surface on whichthe said nickel is deposited and that of chromium.

4. The method in accordance with claim 3, in which said aqueous acidicbath includes a brightening agent enabling the duration of the treatmentto be increased.

5. The method in accordance with claim 4, in which the said brigten-ingagent is selected from the group consisting of the sulphonates,sulphonarnides, and sulphoninsides of aromatic derivatives, butyne 1-4diol, cobalt salts, the amino derivatives of hetcrocyclic compounds suchas Z-aminothiazols, and mixtures of at least two of these agents.

6. The method in accordance with claim 2, in which the said previousmetallic deposit is copper.

7. The method in accordance with claim 2, in which said previousmetallic deposit is brass.

8. The method in accordance with claim 2, in which the said previousmetallic deposit'is selected from the group of electrolytic coatingsconsisting of dull nickel plating, levelling nickel plating,serni-bright nickel plating and bright nickel plating.

9. A method in accordance with claim 8, in which the previous metallicdeposit is a semi-bright or bright nickel plating, and this nickelplating is subjected to a depassivation operation before the deposit ofcracking nickel t-here- 10. The method in accordance with claim 1, inwhich cobalt is deposited simultaneously with said nickel.

11. The method in accordance with claim 1, in which the saidelectrodcposited nickel coating is a duplex nickel plating consisting ofa first deposit of dull or levelling nickel and a second deposit of acoating of cracking nickel.

12, The method in accordance with claim 1, in which said aqueous acidicbath consists essentially of (a) nickel chloride, and (b) a member ofthe group consisting of ammonium acetate, nickel acetate, and mixturesthereof.

13. The method according to claim 1, in which the cracking nickel layeris deposited from said aqueous acid nickel bath at a current density ofbetween 1 and 15 amp/dnfi.

14. The method according to claim 13, wherein the bath contains (a) acompound from the group consisting of nickel chloride, nickel sulphamateand nickel fluoborate and (b) a member of the group consisting of aceticacid, ammonium acetate, nickel acetate and mixtures thereof.

References Cited UNITED STATES PATENTS 2,678,908 5/1954 Tucker 204-4 XR3,282,810 11/1966 Odekerken 204-41 3,298,802 1/1967 Odekerken 20441 XROTHER REFERENCES Lovell, W. E, et al.: Experience in the Operation andPerformance of Dual Chromium Systems, Proc. of the AmericanElectroplaters Society, vol. 47, pp. 215-225, 1960.

Seyb, E. J.: Corrosion Protection with Decorative Chromium, Proc. of theAmerican Electroplaters Society, vol. 47, pp. 269-214, 1960.

Romanotf, F. P.: Ductility and Adhesion of Nickel Deposits, Trans. ofthe Electrochemical society, vol. 65, pp. 385-399, 1934.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner.

